This application relates to the production of anhydrous ethanol, and more particularly to the production of anhydrous ethanol by extractive distillation.
Ethanol is generally produced either synthetically by the hydration of ethylene or by the fermentation of sugar, corn, or other biomass sources. It can be widely used as a chemical intermediate, solvent, and motor-fuel additive.
The Environmental Protection Agency mandated phase-out of tetra-ethyl lead from gasoline and a desire to decrease the dependence on imported crude oil has led to an increased interest in ethanol as a gasoline octane booster and gasoline extender. Furthermore, a recent study indicates that fermentation-ethanol-based ethyl tert-butyl ether has superior properties to the most commonly used oxygenate, methyl tert-butyl ether. The 1986 U.S. production of fuel ethanol from corn was about 600 million gallons, up from nearly zero in 1978, and it is estimated that an additional 5-6 billion gallons per year of ethanol could be produced from the surplus corn grown in the U.S. Because gasohol, gasoline with 10 volume percent ethanol, receives partial exemption from federal excise taxes, at this time ethanol is almost exclusively blended with gasoline in this proportion. Gasohol requires anhydrous (99.5 to 99.8 vol%) ethanol to prevent phase separation of water from gasoline in fuel tanks.
Although other methods exist, the conventional method for producing anhydrous ethanol from dilute fermentation beers or synthetic crude alcohol is by one of two different types of distillation, heterogeneous azeotropic distillation or extractive distillation. Both methods typically use three distillation columns operating at essentially atmospheric pressure with each column heated separately by steam and cooled by cooling water. The first column in each sequence is a preconcentrator often called a stripper-rectifier. The stripping and rectifying may also be accomplished in separate columns. In the first column, a dilute feed containing, for example, 6 to 10 wt% ethanol is concentrated to roughly 85-95 wt% ethanol in the overhead product with the excess water leaving in the bottom stream. The overhead vapor is condensed and part of it is returned as reflux, while the rest is fed to the second or dehydrating tower. At this point, the two distillation methods differ. Heterogeneous azeotropic distillation employs an azeotropic entrainer such as pentane, benzene, heptane, or cyclohexane that forms a heterogeneous ternary azeotrope with ethanol and water. The desired anhydrous ethanol is recovered as the bottom product from this tower. The heterogeneous ternary azeotrope is removed as the overhead product. Upon condensation, the azeotrope phase separates into two liquid phases. The organic phase is used for reflux and the aqueous phase is sent to the third column.
In contrast, extractive distillation uses a high-boiling, completely miscible, nonazeotrope-forming entrainer such as ethylene glycol. The entrainer is used as the upper feed while the preconcentrated ethanol stream becomes the lower feed. The desired anhydrous ethanol product is removed as the distillate and the remaining water and the entrainer exit for the third column via the bottom stream.
The third column in both sequences is used to recover the azeotropic agent or extractive entrainer for recycling to the dehydrating column.
A frequently cited drawback to the production of fuel-grade anhydrous ethanol from renewable grain and biomass sources is that the traditional purification of ethanol by distillation is energy intensive, consuming 50 to 80% of the energy used in typical ethanol-producing fermentation processes. Consequently, a number of energy saving techniques have been proposed over the years, such as methods employing heat pumps, multiple pressure levels for thermal energy re-use, multiple effects, feed preheating schemes, and non-distillation purification processes.
Energy conservation in the heterogeneous azeotropic distillation process has received much more attention in the literature than energy conservation in the extractive distillation process. For example, U.S. Pat. Nos. 1,822,454 and 1,860,544 proposes operating the preconcentrator at a higher pressure than the azeotropic column so that the preconcentrator's condensing overhead vapors could be used to heat the azeotropic column. They also use the bottom stream from the preconcentrator to preheat dilute feed. More recently, U.S. Pat. No. 4,217,178 teaches the use of the condensing overhead vapors of a pressurized preconcentrator to heat both the azeotropic column and the azeotropic-agent recovery column along with a unique feed preheating scheme for additional energy savings. U.S. Pat. No. 4,161,429 teaches increasing the pressure of the azeotropic column relative to the other columns in the sequence for energy conservation. U.S. Pat. No. 4,372,822 teaches pressurizing both the heterogeneous azeotropic column and the azeotropic-agent recovery column so that their condensing overhead vapors could be used to supply the heat needed by the preconcentrator. An energy efficient heterogeneous azeotropic distillation process taught in U.S. Pat. No. 4,422,903 which reduces the steam consumption to the order of 14 to 18 pounds per U.S. gallon of anhydrous ethanol product (11,760-15,120 Btu/gal), depending on the ethanol concentration of the original feed. To accomplish this, the preconcentrator is divided into two effects operating in parallel and three pressure levels are used. The azeotropic and azeotropic-agentrecovery columns operate at the highest pressure level. Their overhead vapors are condensed by reboiling the intermediate-pressure preconcentrator. The condensing overhead vapors of the intermediate-pressure preconcentrator supply the necessary heat to the reboiler of the low-pressure preconcentrator. Additional energy is saved by preheating the dilute feed first with the overhead vapors of the low-pressure preconcentrator and then with the bottom product from both preconcentrators.
In contrast, there is only one study in the literature on thermally-integrated extractive distillation for purifying ethanol. Lynn and Hanson (Ind. Eng. Chem. Proc. Des. Dev., 25, 936, 1986) propose two multi-effect, multiple-pressure-level extractive distillation processes. Both sequences employ two vacuum columns, a vapor feed to the extractive column, and feed preheating by heat exchange with the bottom streams of the preconcentrators. The first alternative uses two preconcentrators operating in parallel. The pressures of one of the preconcentrators and the extractive column are increased so that their condensing overhead vapors reboil the low-pressure preconcentrator. The second alternative splits the preconcentrating step between three effects operating in parallel. The pressures of the intermediate-pressure preconcentrator and the extractive column are sufficiently high that their condensing overhead vapors can be used to supply the required heat to the low-pressure preconcentrator. The pressure of the third preconcentrator is increased enough that its condensing overhead vapors can be used to reboil the intermediate-pressure preconcentrator.
Though the heterogeneous azeotropic distillation process is more commonly used, it does have some serious disadvantages. It is well known in the chemical industry that heterogeneous azeotropic distillations are exceptionally difficult processes to operate and control. Azeotropic columns often behave erratically, and within certain parameters ranges, these columns appear to exhibit multiple steady-states.